The invention relates to a device for determining the state of ageing of an exhaust-gas catalytic converter, and to a method used for this purpose.
German Patent DE 41 12 479 C2 (corresponding to U.S. Pat. No. 5,303,580) describes a method in which the state of ageing of an exhaust-gas catalytic converter is determined with the aid of an oxygen sensor connected to an electronic control unit. The sensor has an oxygen-sensitive region for measuring the oxygen partial pressure in the exhaust gas. The state of ageing of the catalytic converter is determined by the control unit by means of this measured variable.
By contrast, it is an object of the invention to provide a device and a method which allow reliable determination of the state of ageing of an exhaust-gas catalytic converter using simple apparatus.
According to the invention, this object is achieved by a device having an oxygen sensor which is arranged in the exhaust pipe and is assigned to the exhaust-gas catalytic converter, the oxygen sensor having an oxygen-sensitive region for measuring an oxygen partial pressure in the exhaust gas and being connected to an electronic control unit, characterized in that the oxygen sensor has a temperature-sensitive region and can be actuated by the control unit in such a manner that a temperature measurement and/or an oxygen partial pressure measurement can be carried out.
The device according to the invention is distinguished by the fact that the oxygen sensor has a temperature-sensitive region and can be actuated by the control unit in such a manner that a temperature measurement and, as an alternative, the oxygen partial pressure measurement can be carried out. The temperature-sensitive region is in this case at least partially in contact with the exhaust gas. The extent of release of heat of reaction brought about by the catalytic converter can be determined by means of the temperature measurement. Furthermore, the sensor, via the oxygen-sensitive region, can determine the change in the oxygen content in the exhaust gas which is brought about by the catalytic converter. The determination of the two measured variables, temperature and oxygen content, can be carried out as alternatives. It is preferable for the temperature measurement to be carried out when the internal combustion engine is warming up and for the oxygen partial pressure measurement to be carried out when the internal combustion engine has warmed up. The two measured variables are characteristic of various performance features of the catalytic converter, so that its performance and state of ageing can be conclusively ascertained. With regard to the determination of the state of ageing of the catalytic converter, the temperature measurement is preferably used to determine the light-off temperature of the catalytic converter. In the present context, the term light-off temperature is understood, as is customary, to mean the temperature at which the catalytic converter reaches a significant catalytic activity, for example 50% conversion. A low light-off temperature is generally desirable, but this may rise over the course of time during which the catalytic converter is used as a result of ageing. The measurement of the oxygen partial pressure is preferably used to determine the oxygen storage capacity of the catalytic converter. The oxygen storage capacity is likewise subject to ageing. Since the invention allows both the light-off temperature and the oxygen storage capacity of the catalytic converter to be determined, the state of ageing of the catalytic converter can be determined comprehensively and reliably. Furthermore, the measurement of the oxygen partial pressure is used to control the air/fuel ratio (λ) of the air/fuel mixture supplied to the internal combustion engine. Therefore, the oxygen sensor performs a dual function, so that the device can be of simple design.
In one embodiment of the invention, the temperature-sensitive region of the oxygen sensor is formed by its oxygen-sensitive region and is designed in particular as a solid electrolyte. With this configuration, the invention can be realized by means of an electrochemical sensor. The solid electrolyte is used on the one hand to measure the temperature and on the other hand, as an alternative, to measure the oxygen partial pressure in the exhaust gas, with the same sensor part being used for both jobs. It is preferable for the electrical conductivity of the solid electrolyte to be evaluated in order to measure the temperature and for the electromotive force of the Nernst voltage of the solid electrolyte to be evaluated for the purpose of measuring the oxygen partial pressure. This dual function of the solid electrolyte or the oxygen-sensitive region makes it possible to dispense with additional sensor components, resulting in a simple design of sensor.
In a further embodiment of the invention, the temperature-sensitive region of the oxygen sensor is designed as a heating conductor structure. In this way, the heating conductor structure which is generally already present in an oxygen sensor is advantageously used to measure the temperature. It is preferable for the electrical conductivity of this heating conductor structure to be used for the temperature measurement. For this purpose, the material used for the heating conductor structure can be a material which has a relatively high temperature coefficient of its electrical conductivity, so that a substantial measuring effect is achieved. With this configuration of the invention, it is likewise possible to dispense with additional sensor components and a simple design of sensor likewise results.
In a further embodiment of the invention, a temperature probe is provided in the exhaust pipe, and the temperature probe and the oxygen sensor are arranged in such a manner in the exhaust pipe that at least a partial region of the exhaust-gas catalytic converter is located between the oxygen sensor and the temperature probe. This makes it possible to measure a local temperature difference, so that the amount of heat release caused by exothermic reactions in the catalytic converter region can be determined particularly reliably. Consequently, it is also possible for the light-off temperature of the catalytic converter and/or its ageing-induced deterioration to be determined with particular reliability.
In a further embodiment of the invention, the oxygen sensor is arranged in the exhaust-gas catalytic converter or in the exhaust pipe downstream of the exhaust-gas catalytic converter, and a second oxygen sensor is arranged in the exhaust pipe upstream of the exhaust-gas catalytic converter. This makes it possible to measure a local difference in the oxygen partial pressure in the exhaust gas, so that the oxygen storage capacity of the catalytic converter and/or its ageing-induced deterioration can be determined reliably.
The method according to the invention is distinguished by the fact that as the internal combustion engine is warming up, the electrical conductivity of a conductor structure of the oxygen sensor is measured, a first exhaust-gas temperature is determined from this measurement, and the first exhaust-gas temperature is compared with a second exhaust-gas temperature. A component of the sensor which is of relevance to the measurement of the oxygen partial pressure is preferably used to determine the first exhaust-gas temperature. The exhaust-gas temperature measurement carried out by means of this sensor component and the comparison with a second exhaust-gas temperature make it possible to determine the light-off temperature of the catalytic converter. The second exhaust-gas temperature is therefore preferably a temperature which takes account of the increase in temperature of the exhaust gas as a result of the catalytic converter lighting off. Since the light-off temperature represents an important performance feature of the catalytic converter, its state of ageing can be determined with regard to its ability to catalyse a reaction at an early time. The state of ageing of the catalytic converter which is characterized by the light-off temperature can be expressed, for example, by an ageing characteristic value. After the internal combustion engine has warmed up or after the light-off temperature of the catalytic converter has been determined, it is preferable for the oxygen sensor to be used to measure the oxygen partial pressure of the internal combustion engine exhaust gas. λ-control for normal operation of the internal combustion engine can then be effected using this measurement.
In one embodiment of the method, to determine the first exhaust-gas temperature the electrical conductivity of a solid electrolyte, which is used to measure the oxygen partial pressure, of the oxygen sensor is measured. This advantageously makes dual use of the sensitive region of the oxygen sensor.
In a further embodiment of the method, to determine the first exhaust-gas temperature the electrical conductivity of a heating conductor structure of the oxygen sensor is measured. Since heating of the oxygen sensor is required for the oxygen partial pressure measurement function, the oxygen sensor is generally provided with a heating conductor structure. Dual use is advantageously made of this component of the oxygen sensor. It is preferable for the temperature to be determined by measuring the conductivity of the heating conductor structure as the internal combustion engine warms up. After it has warmed up or after light-off of the catalytic converter has been determined by means of the temperature determination, operation of the oxygen sensor is switched over to oxygen partial pressure measurement.
In a further embodiment of the method, the second exhaust-gas temperature is measured using a temperature probe arranged in the exhaust pipe. With a suitable arrangement of oxygen sensor and temperature probe, it is possible to record and determine an increase in the exhaust-gas temperature which is brought about by the catalytic converter lighting off. It is therefore likewise possible to conclude whether or not the catalytic converter is lighting off late as a result of ageing and to evaluate the level of catalytic converter ageing with regard to the light-off temperature.
In a further embodiment of the method, the second exhaust-gas temperature is measured using a second oxygen sensor. For this purpose, the second oxygen sensor is actuated as described above, and the second exhaust-gas temperature is measured by measuring the conductivity of its oxygen-sensitive region or its heating conductor structure. It is preferable for the second oxygen sensor to be arranged in the exhaust pipe upstream of the exhaust-gas catalytic converter and for the other oxygen sensor to be arranged in the catalytic converter or in the exhaust pipe downstream of the catalytic converter. After light-off of the catalytic converter has been recorded, the oxygen sensors are used to measure the oxygen partial pressure and they can be used, for example, to perform λ-control.
In a further embodiment of the method, the second exhaust-gas temperature is determined by modelling. By way of example, expected values for an exhaust-gas temperature at the location of the oxygen sensor can be stored in a control unit by comparison measurements with an unaged catalytic converter. In this case, it is possible also to take into account the operating point of the internal combustion engine. Conclusions can be drawn as to the light-off temperature and/or the state of ageing of the catalytic converter by comparison with the first exhaust-gas temperature determined using the oxygen sensor.
In a further embodiment of the method, a light-off temperature of the catalytic converter is determined from the comparison of the first exhaust-gas temperature and the second exhaust-gas temperature, and an oxygen storage capacity of the catalytic converter is determined from the measurement of the oxygen partial pressure, and the state of ageing of the catalytic converter is determined from the light-off temperature and the oxygen storage capacity. By way of example, the state of ageing of the catalytic converter, as characterized by the light-off temperature, can be expressed by a first ageing characteristic value, and the state of ageing of the catalytic converter, as characterized by the oxygen storage capacity, can be expressed by a second ageing characteristic value. The ageing characteristic values can then be levelled out or compared with one another, so that the state of ageing of the catalytic converter can be analysed more reliably and more comprehensively.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.